#include <bits/stdc++.h>
using namespace std;
int n,c,i,ma,t,nr,mi,st,d,r,j,Ma,mma,tt,nrr;
pair<int,int>v[2010];
int main()
{
    ifstream f ("carnati.in");
    ofstream g ("carnati.out");
    f>>n>>c;
    for(i=1; i<=n; ++i)
        f>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    for(i=1; i<=n; ++i)
    {
        nr=0;
        t=0;
        ma=0;
        mma=0;
        for(j=1; j<=n; ++j)
            if(i!=j)
            {
                if(v[j].second>=v[i].second)
                {
                    if(t==0)t=v[j].first;
                    ++nr;
                    if(mma!=0)++nrr;
                }
                else continue;
                if(ma<nr*v[i].second-(v[j].first-t+1)*c)ma=nr*v[i].second-(v[j].first-t+1)*c;
                else
                {
                    --nr;
                }
                if(mma<nrr*v[i].second-(v[j].first-tt+1)*c)mma=nrr*v[i].second-(v[j].first-tt+1)*c;
                else
                {
                    --nrr;
                }
            }
            else
            {
                mma=v[i].second-c;
                nrr=1;
                tt=v[i].first;
            }
        Ma=max(Ma,ma);
        Ma=max(Ma,mma);
    }
    g<<Ma<<'\n';
    return 0;
}